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POWER GENERATION AND TRANSMISSION
various services range from 1.0 upward. The lumen or unit of light flux is the quantity of light flowing outward from a source of 1 cp. in the solid angle subtended by H2.5664 the surface of a sphere. If the average candlepower in all directions is Ia (mean spherical candlepower) the lumens flowing are 12.56647*. Lumens required = EA/F where A = area of room or surface, square feet, F = factor of utilization. The value of F depends on the shape of the room, absorptions and reflections, and on the variation in candlepower of the light source with direction. It may be as low as 10 per cent for indirect lighting1 and up to 65 per cent for bare lamps in lightly tinted rooms.
Type Watts Lumens Life, hours
Carbon ..... ............ 20-120 50-419 600-2,000
Gem ........................ 20-100 52-422 650-1,000
Mazda (vacuum tungsten) ............. 10-500 70- 5,089 500-1,500
Mazda (gas-filled tungsten) 750-1 , 000 13,000-19 000
Series (vacuum), 4 amperes1 35-350 318- 3,563
Series (gas-filled) ...... 57-500 800-10,000
1 Also made for 3.5, 5.5, 6.6 and 7.5 amp. in the same sizes.
In general, alternating-current arc lamps are less efficient than direct-current, and 220-volt lamps are to be avoided. Enclosed arcs have longer life between trims and greater steadiness, but the efficiency is less. Flaming arcs have a short interval between trims and must be hung high. Luminous arcs give off brown fumes, may be used only outdoors, and give an unsteady light. Efficiency is measured by lumens per watt, or (inversely) by watts per candlepower, usually (as tabulated) the mean candle power of the lower hemisphere.
PURCHASE OF POWER
Water for power is available in some localities. It is usually sold at a fixed price per year for all the water that can flow through an opening of stated dimensions above which an agreed head is to be maintained. The buyer pays whether he uses the water or not. For small installations, the water consumed may be measured by a Venturi meter. The disposition of tail water is sometimes a factor.
Steam (or other) shaft power is furnished as a part of the rental privilege in some cases. In the case of steam the average power demand is estimated by taking indicator cards at the prime mover, with and without the power load in question. Steam as such may be sold by steam meter or (for heating) on the basis of the cubic feet or square feet of space rented.
Electric power is the form commonly sold. The purchaser is apt to overestimate his average load (basing it on his installed capacity) and to underestimate the installation costs incidental to the purchase of power. Annual rates are sometimes made (especially for hydroelectric power) for 10-hr, or
1 In direct lighting, the lamps are exposed. In indirect lighting, no lamps are visible, their light being reflected from walls or ceilings. In the direct-indirect system, part of the light is directed down-